> ## Documentation Index
> Fetch the complete documentation index at: https://www.pgschema.com/llms.txt
> Use this file to discover all available pages before exploring further.

# CREATE DOMAIN

## Syntax

```sql theme={null}
create_domain ::= CREATE DOMAIN domain_name AS data_type
                  [ DEFAULT expression ]
                  [ NOT NULL ]
                  [ constraint_definition [, ...] ]

domain_name ::= [schema.]name

constraint_definition ::= CHECK ( expression )
                        | CONSTRAINT constraint_name CHECK ( expression )
```

pgschema understands the following `CREATE DOMAIN` features:

* **Schema-qualified names**: Domains can be defined in specific schemas
* **Base types**: Any valid PostgreSQL data type as the underlying type
* **Default values**: Default expressions for the domain
* **NOT NULL constraints**: Enforce non-null values
* **CHECK constraints**:
  * Anonymous CHECK constraints
  * Named constraints with CONSTRAINT clause
  * Multiple constraints per domain
  * Expressions using VALUE keyword to reference the domain value

## Canonical Format

When generating migration SQL, pgschema produces domains in the following canonical format:

```sql theme={null}
-- Simple domain without constraints
CREATE DOMAIN [schema.]domain_name AS data_type;

-- Domain with constraints (multi-line format)
CREATE DOMAIN [schema.]domain_name AS data_type
  [ DEFAULT expression ]
  [ NOT NULL ]
  [ CONSTRAINT constraint_name CHECK ( expression ) ]
  [ CHECK ( expression ) ];
```

**Key characteristics of the canonical format:**

* Uses single-line format for simple domains without constraints
* Uses multi-line format with indentation when domain has constraints, defaults, or NOT NULL
* DEFAULT clause appears before NOT NULL when both are present
* Named constraints use `CONSTRAINT constraint_name` prefix
* Anonymous constraints are listed without the CONSTRAINT keyword
* Each constraint is on its own indented line for readability
* For DROP operations: `DROP DOMAIN IF EXISTS domain_name RESTRICT;`

**Note**: Domain modifications in pgschema currently require dropping and recreating the domain, as PostgreSQL has limited ALTER DOMAIN support and complex dependencies make in-place modifications challenging.